Competition for perches between larval damselflies: the influence of perch use on feeding efficiency, growth rate and predator avoidance

SUMMARY. 1. In an investigation of perch use by two species of larval damselfies, Coenagrion puella (Linn.) and Ischnura elegans (van der Linden), larvae spaced out on a grid of vertical cocktail sticks, in the absence of food, to a greater extent than expected from a random model. Spacing was the result of direct interaction between larvae, and was not related to the level of hunger of the larvae over a 10 day starvation period.
2. Prior ownership of a perch was not a predictor of success in conflicts between larvae of the same instar. In contests between different instars larger larvae were more likely to win conflicts. Some small larvae appear to remain undetected in the presence of larger conspecifies.
3. At one level of food availability, perch ownership did not influence growth rate or mortality of C. puella or I. etegans. However, it did allow greater prey capture success rate.
4. Coenagrion puella larvae showed more movement in the absence of perches both in terms of actual distance moved and amount of swimming activity. The amount of swimming activity was reduced by addition of a predator (sticklebacks. Gasterosteus aculeatus or Pungitius pungitius) to the tank both in the absence and presence of an alternative food supply for the fish. Duration of individual swimming bouts was unaffected.
5. Sticklebacks attacked C. puella larvae significantly more often in the absence of perches, with a strong bias towards swimming larvae.
6. It is proposed that the major advantage of perch possession to C. puella larvae is in the reduction of predation as a consequence of the reduction in larval movement.